1. Fields of the Invention
The present invention relates to an anode of solid oxide fuel cell and a method for manufacturing the same, especially to a nanostructured anode of solid oxide fuel cell with high stability of operation and high efficiency of power generation and a method for manufacturing the same.
2. Descriptions of Related Art
Solid oxide fuel cells (SOFC) are devices that utilize electrochemical reactions to generate electric power. In the SOFC operation, generally, oxygen (or air) and hydrogen are used to produce electric power and water. This kind of fuel cells is efficient and free of pollution. Due to environmental issues such as oil depletion and global warming, using the solid oxide fuel cell system as a new clean alternative energy source has become a commonly progressive choice for developed countries.
The SOFC consisted of solid components can be classified into two groups, i.e. planar and tubular type. The energy densities per volume of planar type SOFC are higher than those of tubular type. Thus the planar type has received much attention and become the core of the study. For planar type SOFC, enough mechanical strength is required to support the stacked cells. According to the support material of cell, the planar type SOFC is divided into four subtypes-anode supported, electrolyte supported, cathode supported and metal supported.
The electrolyte supported SOFC has features of relatively higher stability (low degradation rate) and lower energy densities (lower efficiency). Thus anode supported intermediate temperature SOFC with the cermet support structure gets an increased attention in the world. The cermet support of anode supported solid oxide fuel cell is a uniform mixture of ceramic with metal, such as YSZ (yttria stabilized zirconia) with Ni. However, the cermet has issues of high cost, difficult to process, fragile, and having relatively low thermal shock resistance and low thermal conductivity. Toward lowing operation temperatures of SOFC, as being pointed out by Wang et al., “Dynamic evaluation of low-temperature metal-supported solid oxide fuel cell oriented to auxiliary power units,” J. Power Sources, 176, 90, 2008; Tucker, “Progress in metal-supported solid oxide fuel cells: A review,” J. Power Sources, 195, 4570, 2010 and Hwang et al., “High performance metal-supported intermediate temperature solid oxide fuel cells fabricated by atmospheric plasma spraying,” J. Power Sources, 196, 1932, 2011, the planar metal-supported SOFC has received growing attentions and there is a tendency to shift ceramic-supported fuel cells to metal-supported fuel cells, due to the potential benefits of low cost, high strength, better workability, good thermal conductivity and quicker start-up. No matter which type of SOFC is developed, major goals of the current research are to reach the requirements of low cost, high efficiency electric power generation, and high stability of operation.
For a solid oxide fuel cell, Yttria-stabilized zirconia (YSZ) is commonly used as the electrolyte material, Ni/YSZ is currently the most common anode material, and LSM (lanthanum strontium-doped manganite) with perovskite structure is the most currently used cathode material. However, since yttria-stabilized zirconia (YSZ) exhibits sufficient ion conductivity only at high temperatures within a range from 900 to 1000° C., the solid oxide fuel cell made from high-temperature materials with high cost is thus not widely used.
Therefore, in the prior art such as Xu et al., “YSZ thin films deposited by spin-coating for IT-SOFCs,” Ceramics International, 31, 1061, 2005, a thinner yttria-stabilized zirconia (YSZ) electrolyte layer about or less than 5 μm is provided to reduce the resistance and loss under the working temperature lower than 900° C. Alternatively, an electrolyte made of lanthanum strontium gallate magnesite (LSGM) with high ion conductivity can be used to manufacture a solid oxide fuel cell that works at intermediate temperature (600 to 800° C.) with lower manufacturing cost. As the operating temperature of the SOFC system is reduced, the reliability and durability of the SOFC system are significantly improved so that it is helpful to make the SOFC system more acceptably used in home and car applications.
However, when the working temperature of the SOFC is reduced, electrochemical activities of the cathode and the anode also decrease. Thus, the polarization resistances of cathode and anode increase and so does the energy loss. New materials for the cathode and the anode, such as Sm0.5Sr0.5CoO3-δ cathode material and anode composite material containing nickel and LDC (Lanthanum doped Ceria), are required. Moreover, in the prior art, micron-structured cathode and anode are mostly given, these cathode and anode should be changed into having nano-structured features so as to increase the number of triple-phase boundaries (TPB) and improve the electrochemical activities of cathode and anode, and then the energy loss of cell can be reduced.
In the prior art, such as Mukhopadhyay et al., “Engineered anode structure for enhanced electrochemical performance of anode-supported planar solid oxide fuel cell,” International J. of Hydrogen Energy, 37, 2522, 2012, it has been proposed that a cermet anode for a low-temperature and high power density SOFC consists of a thin layer having smaller pores and a thick layer having large pores. The diameter of the small pore should be as smaller as possible. Hence, it is preferred to having the nano-scale pores so as to increase the number of triple-phase boundaries (TPB), but no details on nano-structures of this thin layer with nano pores are revealed in that prior art.
Furthermore, a nanostructuted SOFC anode containing a mixture of nano NiO and micron YSZ is disclosed by Wang in “Influence of size of NiO on the electrochemical properties for SOFC anode,” Chemical Journal of Chinese Universities, 2003, it is produced by high pressure pressing, then sintering and hydrogen reducing at high temperatures. Such SOFC anode has advantages of increased TPB and reduced electrode energy loss. But the particle sizes, especially the nano nickel particles, of the anode change and increase, due to the high temperature operation and sintering processes. Once the sizes of particles in the anode increase, the number of TPB is reduced.
The SOFC can be manufactured by a plurality of methods including tape casting, pulsed laser deposition (PLD) and atmospheric plasma spray. In the methods without high-temperature sintering process, the atmospheric plasma spray has a higher deposition rate for forming a film. Moreover, the temperature of atmospheric plasma flame is so high that it melts or semi-melts injected powder materials quickly, and when these molten or semi molten powder materials hit a substrate, a film is formed on the substrate in a very short time. This is beneficial to producing a nanostructured anode, because the sizes of the powder materials almost remain unchanged during the film growth process. It is difficult to achieve this in the high temperature sintering process for forming such anode film.
Refer to U.S. Pat. No. 8,053,142, a nanostructured composite anode with nano gas channels and an atmosphere plasma spray manufacturing method thereof are disclosed. The nanostructured composite anode with nano gas channels increases the number of TPB and reduces the anode resistance. However, in that patent, no concrete measures on how to ensure the long-term stability of a nanostructured composite anode are disclosed.
In the past, the nanoparticles in nanostructured electrodes have been considered to be quite active and can agglomerate or aggregate to form larger particles. This phenomenon affects the stability of the nanostructured electrodes. Thus there is a need to provide an anode for a high stability and high efficiency solid oxide fuel cell, and a method for manufacturing the same. The operated solid oxide fuel cell with such an anode can have high efficiency and long-term stability at the intermediate temperature range (600˜800° C.).